1. Field of the Invention
The present invention relates to low voltage reset circuit and, more particularly, to a low voltage reset circuit device without being influenced by temperature and manufacturing process.
2. Description of Related Art
FIG. 7 shows a conventional low voltage reset circuit, in which the base-embitter junction voltage (Vbe) of a BJT transistor 71 is used as a reference voltage. The resistors R2 and R3 are used to divide the voltage VDD so as to acquire a desired voltage being a determined ratio of the voltage source. A comparator 72 is used to compare the two voltages for generating a low voltage reset signal. This conventional circuit has two drawbacks. One is that the temperature coefficient of the Vbe is large and thus the operating temperature will negatively influence the detected voltage. The other drawback is that the resistors R2 and R3 will establish a direct current path. The voltage across the R2 and R3 is VDD. The current flowing through the R2 and R3 is I=VDD/(R2+R3). If R2 and R3 are small (for example 100 k) and VDD is assumed to be 3V, this path will cause a power consumption of 30 xcexcA. If the resistor R2 and R3 are increased to be, for example, 500K for saving power, the resistors will occupy a large layout area in the integrated circuit, which results in a high implementation cost (in the 0.6 xcexcm CMOS manufacturing process, the N-well is frequently used as a resistor and, in average, a resistor of 1K occupies an area of 12 xcexcmxc3x9712 xcexcm).
In order to avoid that the temperature effect causes a shift of the reset voltage, as shown in FIG. 8, an energy gap circuit 81 can be used to replace the BJT transistor to generate a reference voltage, and the voltage VDD is so divided through resistors R2 and R3 so as to acquire a voltage being a determined ratio of the voltage source. A comparator 82 is provide to compare the two voltages so as to generate a low voltage reset sign. This conventional circuit can be used to avoid the possible shift caused by the temperature. However, the problems of high power consumption due to the voltage dividing of the resistors and large resistor layout area are not eliminated. Therefore, it is desirable to provide an improved low voltage reset circuit without being affected by temperature and manufacturing process to mitigate and/or obviate the aforementioned problems.
The object of the present invention is to provide a low voltage reset circuit device without being influenced by temperature and manufacturing process, in which the detection of voltage range is complete, the reference voltage to be compared is accurate, and the resistor is small so as not to occupy too much layout area in the integrated circuit.
To achieve the object, there is provided a low voltage reset circuit device for detecting a voltage source to automatically generate a system low voltage reset signal. The low voltage reset circuit comprising: a first low voltage reset circuit using an energy gap circuit to generate a reference, voltage for being compared with the voltage source, so as to output a first reset signal when the voltage source is smaller than a first voltage; a second low voltage reset circuit using a threshold voltage of a MOS transistor as a reference voltage to be compared with the voltage source, so as to generate a second reset signal when the voltage source is smaller than a second voltage, wherein the second voltage is between the first voltage and a lowest voltage at which the energy gap circuit can operate; and a logic circuit for generating a system low voltage reset signal when one of the low voltage reset circuits generates a reset signal.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.